1. Field of the Invention
The present invention relates to a wafer processing method to divide a wafer on which plural planned dividing lines are formed in a lattice manner on the front surface and devices are formed in plural areas partitioned by the plural planned dividing lines, into the individual devices along the planned dividing lines.
2. Description of the Related Art
In a semiconductor device manufacturing process, plural areas are partitioned by planned dividing lines arranged in a lattice manner on a surface of a semiconductor wafer having a substantially circular disc shape and devices such as ICs and LSIs are formed in these partitioned areas. Then, the semiconductor wafer is cut along the planned dividing lines to separate the areas in which the devices are formed, so that the individual devices are manufactured.
Furthermore, in an optical device manufacturing process, optical device layers formed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer are stacked over a surface of a sapphire substrate or a silicon carbide substrate and optical devices such as light emitting diodes and laser diodes are formed in plural areas partitioned by plural planned dividing lines formed in a lattice manner to form an optical device wafer. Then, the optical device wafer is cut along the planned dividing lines to separate the areas in which the optical devices are formed, so that the individual optical devices are manufactured.
As a method for dividing the wafer such as the above-described semiconductor wafer and optical device wafer, a laser processing method has been put into practical use in which a pulse laser beam with such a wavelength as to be transmitted through the wafer is used and the wafer is irradiated with the pulse laser beam with the light focal point positioned inside the area along which the wafer should be divided. The dividing method using this laser processing method is a technique in which a wafer is irradiated with a pulse laser beam with such a wavelength as to be transmitted through the wafer with the light focal point positioned inside from one surface side of the wafer to thereby continuously form a modified layer inside the processing-target object along the planned dividing lines and an external force is applied along the planned dividing lines, on which the strength is lowered due to the formation of this modified layer, to thereby divide the wafer (refer to e.g. Japanese Patent No. 3408805).
In the technique described in Japanese Patent No. 3408805, the light focal point of the laser beam needs to be positioned inside the wafer. Therefore, a protective tape is attached to the front surface, on which devices are formed. Furthermore, the protective tape side is held on a chuck table of the laser beam and the wafer is irradiated with the laser beam from the back surface side of the wafer. In the case of forming plural modified layers in a multi-layering manner, the irradiation is performed with the light focal point of the laser beam positioned near the front surface at first and then the modified layers are formed in a multi-layered manner with the sequential movement of the light focal point toward the back surface side (refer to e.g. Japanese Patent Laid-open No. 2009-140947).
Forming the modified layers in a multi-layering manner with the sequential movement of the light focal point from the front surface side of the wafer toward the back surface side in this manner is based on a thought that, if the modified layer is formed near the back surface at first, the modified layer already formed inhibits the irradiation with the laser beam when the light focal point of the laser beam is brought closer to the front surface.